Method for coating steel strip



W. S. ALLEN METHOD FOR COATING STEEL STRIP Oct. 4, 1955 Filed Jan. 26, 1951 2 Sheets-Sheet l Q w w Q Q n.- o. WH., .r wNv l I @NM NN NN .--I um um n Il im x i: w S w www qm a wr .Q M ,S m @W l @y ,NN Q W I -..wzl

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METHOD FOR COATING STEEL STRIP Filed Jan. 2e, 1951 I 2 sheetsneet 2 l FIG-2- Inventof': ifm/@M 5. #uf/V,

United States Patent O METHOD FOR COATING STEEL STRIP William S. Allen, Pittsburgh, Pa., assignor to United States Steel Corporation, a corporation of New `lersey Application January 26, 1951, Serial No. 207,981

4 Claims. (Cl. 24M-2S) This invention relates to improved methods and apparatus for coating steel strip with tin or other metals.

Electrolytic methods of tinning sheet steel presently are restricted to coatings of 0.75 pound per base box and under, since hot-dip methods are considered more economical for heavier coatings. Considerable savings could be obtained if hot dip methods could be applied to continuous strip operation, but, even though much effort and experimental development has been devoted to this problem, there has been no satisfactory answer heretofore of which I am aware. The greatest difliculty is the removal of the excess tin that always is applied when the strip passes through a bath of molten tin.

An object of the invention is to provide improved tinning methods and apparatus which are capable of depositing coatings of any desired weight from about 0.5 to 3.0 pounds or heavier per base box on continuous lengths of steel strip.

A further object of the invention is to provide improved tinning methods and apparatus which deposit on steel strip only the amount of tin desired on the finished product.

A further object of the invention is to provide improved coating methods and apparatus which combine electrolytic coating with spray coating to enable a metal coating of any desired weight to be built up on continuous lengths of steel strip.

A further object of the invention is to provide improved apparatus and methods for applying metal coatings to steel strip in which a continuous strip first receives a lightweight prime coating electrolytically and thereafter receives a coating of any desired weight by spraying, the two coatingsV subsequently being melted into a unified coating alloyed with the steel.

In accomplishing these and other objects of the invention, I have provided improved details of structure, a preferred form of which is shown in the accompanying drawings, in which:

Figure l is a side elevational view, in which some details are shown schematically, of a portion of a metal coating apparatus embodying features of the present invention; n

Figure 2 is a vertical sectional view of the spray device employed in the apparatus; and

Figure 3 is an end elevational View of the device shown in Figure 2.

Figure l shows a portion. of a metal coating line which, in accordance with the present invention, includes in combination an electrolytic cell and a spray coating mechanism 12. A continuous steel strip S travels from left to right, passing first through the electrolytic cell and subsequently through the spray coating mechanism. Before reaching the former, the strip is cleaned and pickled ink the usual manner. After discharging from the latter, the coated strip can be subjected to any of the usual finishing operations, such as surface chemical treatment and/ or oiling.

The electrolytic cell 10 illustrated comprises a tank 13, conductor rolls 14, a sink roll and anodes 16, which in the present example are tin. The tank contains any desired plating bath from which a metal coating, in this instance tin, can be deposited electrolytically on the strip S, as known in the art. Since the electrolytic cell need deposit only a very lightweight coating on the strip (0.1 to 0.2 pound per base box) only a single cell is sufficient.

Between the electrolytic cell 10 and the spray coating mechanism 12, the strip passes downwardly through a drag-out recovery tank 17, around a sink roll 18 in said tank, upwardly through said tank and through a steam drier 19, over a grounded conductor roll 20 and downwardly through a choke core 21. The strip then passes around two conductor rolls 22 and 23, which are situated at opposite ends of the spray mechanism, hereinafter described, and which introduce alternating current to the strip for melting the coating. The purpose of the choke core is to limit the current which can flow back through the strip from roll 22 and the purpose of the ground on roll 20 is to prevent any of this current from entering the electrolytic cell.

The electrolytic cell 10, the various conductor and sink rolls, the drag-out recovery tank 17, the steam drier 19, and the choke core 21 per se can all be of any standard or desired construction. The various rolls can be power driven for propelling the strip in any usual fashion. The use of alternating current for melting the coating and of the choke core and grounded conductor roll for limiting the flow of such current also is as known in the art. Therefore structural features of these parts and their mode of operation are not described in detail.

The spray coating mechanism comprises an enclosed two-pass muie 24 which is made of heat insulating material and has an electrically insulated roll 25 in its upper portion. The strip S travels from the conductor roll 22 upwardly through the first pass of the muflle, over the insulated roll 25, downwardly through the second pass of the mufe, and around the conductor roll 23. The muftle has a mechanical seal 26 at its entry end. The conductor roll 23 is mounted in a tank 27 which contains a water or oil bath that both serves as a liquid seal for the exit end of the mule and also quenches the strip after its coating has melted. The mutlie contains a non-oxidizing or reducing atmosphere and the two seals prevent entry of air.

The second pass of the muflle has a section 28 which is of an electrically non-conductive material, such as brick. Two sets of opposed spray nozzles 29 and 29a of non-corrodible material, such as stainless steel, are mount ed on opposite sides of the mutlle in its section 28. The nozzles of each set can be spaced about 4 inches apart and about 6 to 8 inches from the strip surface. The nozzles are supplied with molten metal, in this instance tin, from individual containers 30 and 30a. Said containers are equippedl with burners 31 and 31a which maintain their contents in a molten state. Compressors 32 and 32a supply the nozzles with a stream of`non-oxidizing atmosphere from the mufile for atomizing the sprays of molten coating metal. Preferablyy heaters 33 and 33a are interposed between the compressors and the nozzles to heat the gas before it reaches the latter.

Figures 2 and 3 show the details of a preferred construction of nozzle 29 and container 30, the other nozzle 29a and container 30a being similar. The nozzle contains a needle valve 34 which has an operating solenoid 35. This valve controls the amount of metal sprayed through the nozzle and also furnishes a convenient shutolf. A gas chamber 36 surrounds the nozzle and has a connection 37 for admitting gas from the compressor 32. The container is equipped with a platform 38 and gate 39 for supplying additional bars T of the coating metal.

The spray mechanism can be equipped with electrostatic means for attracting the molten particles of metal in the Vspray to the strip. Figure l shows'two pairs of transformers and electronic rectiers 40 and 40a, which can be of any standard or desired construction and are connected to low voltage A. C. lines 41 and 42. The rectifiers each have positive andfnegative D.-C. outlet linesk 43 and 44.' The positive outlet lines-43 are grounded and thus are connected to the strip viaA the ground. The nega.-v

tiveoutlet lines' 44 are connected to the containers 30 static charge supplied to the particlesof coating metal viathe transformers and rectitiers 40a yand the spray guns 29 and 29a only. A further possibility is to eliminate the transformers and rectiiiers 40a'and to supply the electrostatic charges to the particles solely by induction as they pass through the grids 4S and 45a.

The atmosphere Within the muflle or else the strip surface,.preferably the former, can include a uxing agent which reduces the surface tension of the particles of coating metal and thus insures a smoother coating. When the furnace atmosphere carries the flux, the flux can be HCl, Clz or SnCl4 vapor. When the strip carries the flux, the flux can be an aqueous solutionof SnClz, or other tin salt, ZnClz or NHrCl or palm oil.

According to the coating method ofthe present inventionthe cleaned and pickled steel strip S traveling at a speed of 100 to 350 feet per minute, passes rst through the electrolytic cell 10, where it receives a lightweight prime coating of 0.1 to 0.2 pound per base box. This step per se is performed in the usual manner except that the coating is of lighter weight than when the electrolytically coated strip is a finished product. Next the strip passes through the drag-out recovery tank 17, the steam drier 19, and the choke core 21, all of which perform their usual function. Next the Strip passes around the conductor roll 22, through the muie 24 of the spray coating mechanism 12 and around the conductor roll 23. rl`hese two conductor rolls apply alternating current to the strip and heat it electrically sufficiently to melt the metal deposit. This melting also isv carried out in the usual manner and the variables are controlled so that the coating attains its melting temperature inthe vicinity of the spray nozzles 29 and 29a.

As the strip passes the nozzles, they spray fine particles of molten coating metal on its surface. The opposite electrostatic charges on the particles and on the strip causes these particles to be attracted to the strip surface. The non-oxidizing or reducing atmosphere of the Inutile prevents surface oxidation of the particles and the linx enables them to spreaduniformly on the strip surface. The weight of coating deposited can be controlled by the opening of the needle vvalve 34, by adjusting the'pressureV ofgasrdelivered from the, compressors 32 and 32a, andV by adjusting the strip Speed. For example at a line speed of 300 feet per'minute and a tin coating of 1.5 pounds perl base box, nozzles spaced 4 inches apart would be required to spray about 0.4 poundgof tin per minute.

About the time the strip passes the nozzles, the coating melts throughout and alloys itself with the steel surface of the strip. The strip then entersy the quenching liquid in tank 27 and the coating again solidies. The coated steel strip then is ready for any desired finishing steps;

The method of the present invention is" intended primarily for tinning, but it also can be used for applying coatings of lead, terne, zinc and other metals. It is also possible to apply coatings in.which the electrolytic prime coat is a different metal from the sprayed coating. By choosing the prime coat so that it is capable of alloying with both the steel and the sprayed coating, it serves both as a bonding agent and insulator between the steel surface and the outer coating. Thus a coating, the outer surface of which is predominantly lead, could be made by spraying molten lead on a thin electrolytic tin prime coat and'.

then melting. Or a zinc coating` havinga thin, moreexible alloy than conventional galvanized coatings, would result from spraying molten zinc on a similar tirr coat and melting.

From the foregoing description, it is seen that the present invention aords a simple and effective apparatus and method for tinning or otherwise coating continuous steel strip in which the coatings can be of any desired weight.

The method and apparatus attains all the advantages-inf V herent in continuous strip coating, not possible previously when heavy coatings were required. The conventional flux pot with its corrosive and toxic fumesis eliminated.y The grease pot also is eliminated, thus greatly simplifying;

cooling and cleaning problems.

While I have shown and described certain preferred` embodiments of the invention, it is apparent that modifications may arise. Therefore, I do not wish to be limited to the disclosure set forth but only by the scope of theappended claims.

I claim:

1. A method of applying to steel strip coatings of metals of the group consisting of tin, terne, lead and zinc com# prising electrolytically applying to the strip a lightweight prime coating of one of said metals,- said prime coating being of a weight equivalent to 0.1 to 0.2 pound per base box, introducing the strip to a sealed'enclosure having-a non-oxidizing atmosphere, spraying a heavier coating of one of said metals on the strip within said enclosure,` heating the strip Within said enclosure and controlling the heating so that the strip surface attains a temperature sut'cient to melt the prime coating and alloythe coat-- ings with the strip surface in the vicinity of the point Where the heavier coating is sprayed thereon, and quenchingpthe strip.

2. A method as defined in claim lV in whichthe metal used in both coatingsis tin.

3. A method as defined in claim 1 in which the' heavier coating is sprayed on the strip in-an electrostatic field.

4. A method as defined in claim 1 in which the strip.v is heated electrically.

References Cited in the le of this patent UNITED STATES PATENTS FOREIGN PATENTS Great Britain Apr. 27, 1923. 

1. A METHOD OF APPLYING TO STEEL STRIP COATINGS OF METALS OF THE GROUP CONSISTING OF TIN, TERNE, LEAD AND ZINC COMPRISING ELECTROLYTICALLY APPLYING TO THE STRIP A LIGHTWEIGHT PRIME COATING OF ONE OF SAID METALS, SAID PRIME COATING BEING OF A WEIGHT EQUIVALENT TO 0.1 TO 0.2 POUND PER BASE BOX, INTRODUCING THE STRIP TO A SEALED ENCLOSURE HAVING A NON-OXIDIZING ATMOSPHERE, SPRAYING A HEAVIER COATING OF ONE SAID METALS ON THE STRIP WITHIN SAID ENCLOSURE, HEATING THE STRIP WITHIN SAID ENCLOSURE AND CONTROLLING THE HEATING SO THAT THE STRIP SURFACE ATTAINS A TEMPERATURE SUFFICIENT TO MELT THE PRIME COATING AND ALLOY THE COATINGS WITH THE STRIP SURFACE IN THE VICINITY OF THE POINT WHERE THE HEAVIER COATING IS SPRAYED THEREON, AND QUENCHING THE STRIP. 